The outbred mammalian fetus by virtue of its genetic complement of paternal transplantation antigens defies the expected laws of transplantation by surviving during gestation without being immunologic rejected. We plan to investigate three aspects of the maternal fetal relationship that bear on this phenomonon. First, we will isolate and characterize serum immunosuppressive factors responsible for inhibiting in vitro lymphocyte responses of the mother against the fetus. Second, we will search for suppressor cells in pregnant animals that negatively regulate the transplantation response of the mother against the fetus. Finally, we will continue to evaluate the role and the mechanism of action of sex steroids, particularly progesterone in abrogating immune responses.